Broadly stated, the goal of any energy saving system is to prolong the functional life of a device without degrading or disrupting the device's proper operation. To do this, the lifetime of the device's energy source (i.e., the battery) must be extended. Several energy saving techniques designed to accomplish battery life extension are known. The most widely known techniques involve the "Battery Savers" commonly used in the paging receiver arts. Typical of these battery saving techniques are methods to temporarily inactivate non-essential circuits during periods of non-activity. In more advanced battery savers, it is also known to reduce the operational speed (i.e., clock rate) of microprocessors and other digital circuits. In this way, the discharge rate of the battery is reduced thereby extending the pager's operational life.
However useful the paging battery saving techniques may be, they suffer a common detriment that mitigates against their proliferation into other applications, such as, for example, a trunked two-way communication system. This detriment consists chiefly of designed-in inflexibility and inefficiency in the communication protocol. Typically, paging systems operate using time division multiplex (TDM) principles wherein a communication channel is divided into repetitive frames having multiple slots. One slot comprises a control/sync slot that each pager must receive to determine if one or more subsequent slots contain a message to be received. Thus, a pager not receiving a message in any particular frame need only be active to receive the control/sync slot, and may "sleep" for the remaining duration of the frame.
While facially sound, the battery saving practices of the paging receiver art are inflexible in that the communication protocol need be preconceived and fixed prior to providing communication services. Once established, protocol designers typically resists modifications or improvements despite technological advances, system loading, or other changed conditions. Moreover, conventional paging battery saving techniques are inefficient in that each pager must receive the control/sync slot whether or not it will receive a message in that frame. The process of receiving the control/sync slot inefficient in and of itself for each frame that a message is not received. Accordingly, extending these techniques into other applications would only continue the proliferation of inflexible communication systems.
In contrast to paging systems, two-way trunked communication systems operate to allocate several communication channels among a plurality of subscriber units. When not participating in a call, each subscriber unit monitors a control channel to receive signalling and control information that permits the subscriber units to communicate either individually or in groups (commonly denoted "talk-groups"). The signalling and control information transmitted over the control channel is repeated from time to time to insure that each subscriber unit will receive the required channel information. In this way, a subscriber unit returning to the control channel from a first call may receive the information required to join another call already in progress. Accordingly, a need exists for an energy saving protocol suitable for use in two-way communication systems.